LeetCode-in-All

6. Zigzag Conversion

Medium

The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility)

P A H N A P L S I I G Y I R

And then read line by line: "PAHNAPLSIIGYIR"

Write the code that will take a string and make this conversion given a number of rows:

string convert(string s, int numRows);

Example 1:

Input: s = “PAYPALISHIRING”, numRows = 3

Output: “PAHNAPLSIIGYIR”

Example 2:

Input: s = “PAYPALISHIRING”, numRows = 4

Output: “PINALSIGYAHRPI”

Explanation: P I N A L S I G Y A H R P I

Example 3:

Input: s = “A”, numRows = 1

Output: “A”

Constraints:

• 1 <= s.length <= 1000
• s consists of English letters (lower-case and upper-case), ',' and '.'.
• 1 <= numRows <= 1000

Solution

(define/contract (convert s numRows)
  (-> string? exact-integer? string?)
  (let* ((sLen (string-length s))
         (maxDist (- (* numRows 2) 2)))
    (if (= numRows 1)
        s
        (let ((buf ""))
          (for ([i (in-range numRows)])
            (let loop ((index i))
              (when (< index sLen)
                (set! buf (string-append buf (string (string-ref s index))))
                (let ((next-step (- maxDist (* i 2))))
                  (when (and (> i 0) (< i (- numRows 1)) (< (+ index next-step) sLen))
                    (set! buf (string-append buf (string (string-ref s (+ index next-step))))))
                  (loop (+ index maxDist))))))
          buf))))

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